Treehouse Handshake/The Big Pictures

Sally invents a secret treehouse handshake, but Nick can't remember the sequence. The Cat takes them to Over-and-Overton, where they figure out how to get through a series of obstacles by recognizing patterns. Nick and Sally don't understand why one toy car goes faster than another. The Cat says he needs their help to move some big pictures and along the way, they find out about how air is matter (takes up space and has mass) and can push on objects.